1. Field of the Invention
The present invention relates to a light emitting display device having light emitting elements, and further relates to a method for driving a light emitting element when an electric field is applied thereto.
2. Description of the Related Art
Due to recent demand for high definition images, a self-light-emitting type of organic electroluminescent light emitting element (hereinafter referred to as xe2x80x9clight emitting elementxe2x80x9d) has become a focus of attention. Due to advancements in organic layer materials, this light emitting element is highly efficient and has long life.
Referring to FIG. 7, a light emitting element E is composed of a metallic electrode 101 (a cathode), a transparent electrode 102 (an anode), an organic compound that is stacked between the electrodes 101 and 102, and a glass substrate 105 arranged outside the transparent electrode 102. The organic compound consists of an organic fluorescent thin film 103 and an organic hole transporting layer 104.
In a light emitting element having the configuration of FIG. 7, an exciter is generated by a recombination between an electron and a hole. The electron is generated in the metallic electrode 101 by a driving source 106, and the hole is from the transparent electrode 102. When the exciter is discharged and deactivated, light is emitted. The emitted light is externally released through the transparent electrode 102 and the glass substrate 105.
The light emitting element E, in which electrodes and organic fluorescent material are stacked, has a parasitic capacitance in its electric-equivalent circuit shown in FIG. 8. In this circuit, reference numeral 107 denotes a light emitting body of a constant voltage element, reference numeral 108 denotes an internal resistance, and reference numeral 109 denotes a parasitic capacitance. The parasitic capacitance 109 is connected in parallel with the light emitting body 107 and internal resistance 108.
FIG. 9 shows a variation in voltage applied to the light emitting element E when driven during a scanning period using a constant current driving technique. The ordinate indicates a voltage applied across the light emitting element E, and the abscissa indicates time. Reference numeral 110 denotes a scanning time, and reference numeral 111 denotes a charging time of the parasitic capacitance 109 of the light emitting element E. Reference symbol Vf denotes a forward voltage during maximum light emission, which depends on the static characteristic of the light emitting body 107.
As can be seen in FIG. 9, after the start of the scanning period, the voltage applied to the light emitting element E does not reach Vf immediately. The delay is due to the current supplied from the driving source initially being consumed to charge the parasitic capacitance 109. The light emitted by the light emitting element E is proportional to the driving current. While the light emitting element E emits light with stable brightness after the parasitic capacitance is charged, the brightness during the initial period is not sufficient. The adverse result is that the brightness varies during the scanning period, and the average brightness over the entire scanning period is reduced.
In view of the problem described above, it is an object of the present invention to provide a light emitting display device which requires a shorter time to emit light with a desired instantaneous brightness and has less variation in instantaneous brightness during a scanning period.
The present invention includes a light emitting display device having a plurality of light emitting elements, comprising first and second driving sources, a connection selector, and a controller. The first and second driving sources are connectable to the light emitting elements. The connection selector selects one of the first and the second driving sources, and connects the selected driving source to the light emitting elements. The controller controls the connection selector to connect the first driving source to the light emitting elements, and subsequently, in exchange for the first driving source, connects the second driving source to the light emitting elements. A driving current supplied to the light emitting elements by the first driving source is larger than a driving current supplied to the light emitting elements by the second driving source.
The first driving source may be a constant voltage source, and the second driving source may be a constant current source. Alternatively, each of the first and the second driving sources may be constant current sources.
The invention also includes a method for driving a light emitting display device having a plurality of light emitting elements. First and second driving sources, which are connectable to the light emitting elements, are provided. Driving currents are supplied to the light emitting elements, wherein a driving current supplied by the first driving source is larger than a driving current supplied by the second driving source. The first driving source is first connected to the light emitting elements. Subsequently and in exchange for the first driving source, the second driving source is connected to the light emitting elements.
The invention further includes a method for driving a light emitting display device having light emitting elements connected to intersecting points of a plurality of anode lines and cathode lines arranged in a matrix. Either one of the anode lines and the cathode lines are used as scanning lines, while the others are used as driving lines. While one of the scanning lines is scanned during a scanning period, a driving source is synchronously connected to one of the driving lines so that a light emitting element connected to an intersecting point of the one scanning line and the one driving line is caused to emit light. Immediately after the scanning period of the one scanning line is started, a first driving source is connected to the one driving line. Subsequently, in exchange for the first driving source, a second driving source is connected to the one driving line.
In order to drive a light emitting element during a scanning period, the parasitic capacitance of a light emitting element can be charged at a high speed by a first driving source and thereafter the light emitting element can be driven with constant instantaneous brightness. Therefore, the time elapsing until the light emitting element emits light with desired instantaneous brightness can be shortened and variation in the instantaneous brightness within a scanning period can be reduced.